The following invention relates to a web slitting machine of the type used for cutting a continuous web of material.
Web slitters are cutting machines commonly employed to cut an endless web, such as a continuous roll of material, into strips. Typical machines of this type include an upper blade portion which overlaps with a lower knife to provide a scissors-like action for cutting a continuous roll as it is pulled between the blade and the knife. The blade is usually a nonpowered rotary cutting disk suspended from a carriage which is attached to a transverse bar. A plurality of web slitters may be connected to the same bar to create parallel strips of various widths. The lower knife may be a blade supported from underneath the roll or may be a roller or drum having a sharpened edge. Together the lower knife and the upper blade create a shearing action against the web as it unwinds from a roll and is pulled through the web slitter by a rewind or take-up roll.
It is important in the design of such machines that the shear or cant angle between the upper blade and lower knife be precisely maintained. The cant angle is the angular relationship between the upper blade and the lower knife in the plane of the blade about a vertical axis. This angle must be maintained so that the wear and deformation between the two cutting edges are kept to a minimum. The cant angle should also be adjustable to compensate for various blade-to-web orientations as well as various types of web material. Rotation of the upper blade about the aforementioned vertical axis results in deviations from the desired cant angle and various approaches have been tried to correct this problem.
One such approach is shown in Markowski U.S. Pat. No. 3,143,024. In the Markowski device, the upper blade is supported on a shaft which is clamped to a support assembly connected to a transverse bar. The shaft includes a milled surface which is held against a tapered block to create the desired cant angle, and the tapered block may be replaced for other blocks of different dimensions, thereby providing adjustability. Another approach is to use a clamp and a knife shaft which are replaceable, so that various cant angles may be obtained by substitution of these parts, an example of which is shown in Waterhouse U.S. Pat. No. 3,186,282. Various types of screw adjustments are shown in Printz, et al., U.S. Pat. No. 3,185,010, Johnstone U.S. Pat. No. 3,892,156 and Aykut U.S. Pat. No. 4,257,299, and differential springs are used for setting the cant angle in Noffke, et al., U.S. Pat. No. 4,438,673.
The problem with all of the aforementioned devices is that the means for holding the blade in a predetermined cant angle position is not strong enough to maintain the angle when the web slitter is subjected to the force of the moving web. Moreover, springs and screws which may be easily adjusted are also subject to tampering. The use of a replaceable tapered block as shown in Markowski alleviates the problem of tampering; however, the block bears against but a small flattened portion of the blade-supporting shaft. Moreover, the pressure holding the tapered block is maintained by a clamp tightened by a knob acting on a threaded shaft. If the shaft becomes loose, the blade will wobble.
Some web slitters mount a blade holder along a track to provide for easy replacement of the blade. An example of this construction is shown in Colombo, U.S. Pat. No. 4,741,234. The track mounting of Colombo, however, is integral to the upper carriage assembly and provides no adjustability for the cant angle. Moreover the blade holder portion is not reversible on the track. Reversibility would be a desirable feature since it is sometimes necessary to mount the carriage for left-hand as well as right-hand operation.
It is critical to the maintaining of proper alignment of the blade and lower knife that the blade-supporting shaft does not rotate. Most shafts are round, but some attempts have been made to keep the shaft from rotating by milling portions of it to a flat shape and providing a bearing pressing against the flattened portion to prevent rotation. An example is shown in the Cavagna EPO Patent Application No. 84/100104.3. Other approaches include a keyed shaft as shown in the Johnstone '156 patent and a trangular-oval piston shown in Wingen, U.S. Pat. No. 3,434,695. The problem with these shapes is that they are either expensive to manufacture, or do not provide sufficient rigidity to keep the shaft from rotating.
Another design aspect of conventional web slitters is that many include pneumatic actuation features for locking the web slitter to a transverse bar, for lowering the blade into a cutting position, and for laterally shifting the blade towards and away from the knife. These functions are usually performed in a predetermined sequence, which may limit the flexibility of any pneumatic control system that may be employed. An example is shown in Cavagna U.S. Pat. No. 4,540,394, in which the actuation of the pneumatic control which locks the upper carriage to a transverse bar also simultaneously lowers a piston supporting the lower blade assembly. With such a system it is not possible either to unlock the carriage from the transverse bar without raising the blade, or to lock the carriage to the transverse bar without lowering the blade. There may exist situations, however, in which it would be desirable to be able to raise the blade without unlocking the carriage, or where it would be desirable to have the blade lowered but have the ability to move the carriage along the transverse bar.
As mentioned above, a plurality of web slitters of the type described herein may be mounted on a transverse bar and may be positioned along the bar at various locations to define the width of strips to be cut from the web. An arrangement for setting the desired transverse location along the bar using a top mounted knob driving a pinion which interacts with a rack on the bar shown in the "System Helios" web slitter manufactured by Tidland Corporation of Camas, Wash. The knob is inconveniently placed, however, and is, for that reason, awkward to use. Also, the sideways movement tends to cause "crabbing" as the web slitter will experience a tilting moment about its horizontal axis as the carriage moves transversely.
Many conventional web slitters include a pneumatic side shift feature which moves the blade laterally into contact with the lower knife. This usually occurs after the blade has been lowered. Examples are shown in the U.S. patents to Johnstone, U.S. Pat. No. 3,892,156 and Gilmore, U.S. Pat. No. 3,380,330. No provision is made, however, in either of these two devices for quickly retracting the blade laterally before the blade is raised. The problem that can occur when the blade is raised without sideways retraction, is that it may scrape across the edge of the lower knife which can cause chipping or dulling.
Web slitters which include a side shift feature as described above also encounter the problem that when the blade shifts, it must contact the lower knife with pressure sufficient to maintain the blade against the lower knife in a good cutting relationship, but not have so much pressure so as to cause the blade to tilt. This requires some guesswork as to where to clamp the carriage portion along the transverse bar.
This problem is inherent in the Johnstone '156 device and in a device shown in the Gilmore '330 patent, both of which include pneumatic side shifting actuators which automatically shift the blade laterally upon completion of the downstroke. A desirable feature in such devices would be a mechanism permitting optimum adjustment of the force of the blade against the lower knife.
Continuous web material which is forced through a gang of web slitters is not always uniform. It frequently contains irregularities which may have a tendency to force the blade away from the lower knife, notwithstanding the pneumatic pressure imparted by the side shifting mechanism. When the web travels at high speed, this becomes a problem since the friction in the piston and cylinder arrangement used in the side shift mechanism prevents it from reacting quickly enough to prevent oscillations of the blade. Thus, it would be desirable to provide a fast-acting biasing or shock absorbing means for high speed moving webs to damp blade oscillation and to create a restoring force that will maintain the blade in proper contact with the lower knife.
The blade portion of a conventional web slitter is a nondriven freely rotating disk. The disks are clamped with screws or the like to a hub and must periodically be replaced when they become dull. This can be a dangerous operation because the operator must usually grasp the blade along its edge to stop it from rotating while a blade retainer or clamp is removed. A desirable feature would be one that prevented blade rotation while it was being replaced.